APPLICATION OF ZIGZAG TRANSFORMERS FOR REDUCING HARMONICS IN THE
NEUTRAL CONDUCTOR OF LOW VOLTAGE DISTRIBUTION SYSTEM
PREM P . KHERA
Member, IEEE
3M Company
F a c i l i t i e s Engineering
P.O. BOX 33331, Bldg. 42-7W-03
S t . Paul, MN 55133, USA
Abstract - A novel application of zigzag transformers
f o r c i n g harmonics in the neutral conductors of a
t h r e e phase, 208/120V d i s t r i b u t i o n system i s presented.
Third harmonic c u r r e n t s add u p in the neutral conductor
of the d i s t r i b u t i o n system feeding non-1 inear loads
such a s personal computers and e l e c t r o n i c o f f i c e
machines with switch mode power supplies. Other
solutions t o t h i s problem include derating t h e s i z e of
t h e d i s t r i b u t i o n transformer by 50%. However,
application of the zigzag transformer eliminates the
need f o r t h i s derating. Measurements of c u r r e n t draw
before and a f t e r t h e i n s t a l l a t i o n of a zigzag transformer a r e presented.
In the past few years switch mode power supplies (SMPS)
have become the power conversion technology used in
personal computers and e l e c t r o n i c o f f i c e machines d u e
t o t h e i r high efficiency and low c o s t compared with
the previously used 1 i n e a r regulator power supplies.
One problem with SMPS i s t h a t i t s n o n l i n e a r i t i e s give
r i s e t o harmonics. When a l a r g e number of s i n g l e phase
120V devices with SMPS a r e powered from a three phase
208/120V d i s t r i b u t i o n system, t h e amount of harmonics
generated a r e high enough t o cause overheating of the
d i s t r i b u t i o n transformer. In addition, the neutral
conductor s u f f e r s from overheating because t h e t h i r d
harmonic c u r r e n t s from the three phases do not cancel
within t h e conductor.
One possible solution t o overcome the heating e f f e c t
of harmonic currents i s t o d e r a t e the e x i s t i n g
d i s t r i b u t i o n transformer t o a s low a s 50% of i t s rating.
Another solution i s t o add a zigzag transformer in the
d i s t r i b u t i o n system. The zigzag transformer has been
used in the past f o r creating a n e u t r a l , thereby
converting a t h r e e phase, three wire d i s t r i b u t i o n
system t o a t h r e e phase, f o u r wire system. The
schematic diagram of the zigzag transformer i s shown
in f i g u r e 1 and the vector diagram in f i g u r e 2 .
.
-
-I L PHASE A
IT
-
FOR BRANCH CIRCUIT
PANEL FEEDINQ
NON-LINEAR LOAD
1zonOBv.
FROM
'\
DlSTRlBUTlON
TRANSFORMER
B-tI+puE
PHASE C
FIG 2 VECTOR DIAGRAM
ZIGZAG
WSTALLATKW
The application presented here does not involve the
creation of a neutral b u t r a t h e r uses t h e zigzag
transformer neutral t o share the load neutral current.
The amount of t h i s sharing depends on the impedance
of the a l t e r n a t e c u r r e n t paths.
--.-
U_
As shown in f i g u r e 3, three 5KVA, 120 X 240 120/240V, dry type transformers were wired in a zigzag
fashion. A 20-turnY 8 inch diameter c o i l in a i r was
chosen t o introduce an impedance in the n e u t r a l , thus
allowing t h e c u r r e n t t o s p l i t i n t o two paths, one t o
the d i s t r i b u t i o n transformer, the other t o the zigzag
transformer. Current measurements were done using
t r u e RMS meter. Before the zigzag transformer
i n s t a l l a t i o n , the phase load c u r r e n t s were averaged
a t 89A. Excluding the load unbalance on the three
phases, t h e neutral harmonic current was 102A. After
t h e i n s t a l l a t i o n , t h e c u r r e n t in the d i s t r i b u t i o n
transformer neutral was 54 Amps, a reduction of 47%.
Also, the t r u e power f a c t o r was measured t o be 91%
and the displacement f a c t o r 96%.
The introduction of the 20-turn c o i l impedance in the
neutral conductor lowered the f a u l t c u r r e n t on a
l i n e t o neutral f a u l t , b u t did not a f f e c t the
instantaneous tripping of the c i r c u i t breaker in the
event of a f a u l t . The c o i l impedance was calculated
t o be 0.0112 Ohms. More turns could be added in the
c o i l t o f u r t h e r reduce the c u r r e n t in the
d i s t r i b u t i o n transformer neutral and not a f f e c t the
instantaneous tripping of t h e c i r c u i t breaker.
Ref
- erencks
Computer Business Equipment Manufacturers
Associati o n ' s Informati on L e t t e r , December 1988.
University of Wisconsin course on "Effects of
Non-1 inear Loads on the Power Distribution System"
April 5 - 6 , 1989.
90/CH 2935-5/90/0000-1092$01.0001990 IEEE